Microphone assembly with digital feedback loop

ABSTRACT

A microphone assembly includes a transducer and a processing circuit. The processing circuit includes an analog-to-digital converter (ADC) configured to receive, sample and quantize an electrical signal generated by the transducer to generate a corresponding digital signal. The processing circuit includes a feedback path including a digital loop filter configured to receive and filter the digital signal to provide a first digital feedback signal and a digital-to-analog converter (DAC) configured to convert the first digital feedback signal into a corresponding analog feedback signal. The processing circuit additionally includes a summing node configured to combine the electrical signal and the analog feedback signal.

BACKGROUND

Portable communication and computing devices such as smartphones, mobilephones, tablets etc. are compact devices which are powered fromrechargeable battery sources. The compact dimensions and battery sourceput severe constraints on the maximum acceptable dimensions and powerconsumption of microphones and microphone amplification circuit utilizedin such portable communication devices.

US 2011/0051954 A1 discloses a signal conditioner for a capacitivetransducer. The signal conditioner includes a differential preamplifierwhich includes a non-inverting input connected to a microphonetransducer output and a separate inverting input connected to the outputof a feedback path. The feedback path includes a digital-to-analogueconverter and extends from an output of an analogue-to-digital converterto the inverting input of the preamplifier.

However, there exists a continued need to improve the sound quality androbustness of microphone assemblies for example by accuratelycontrolling the frequency response and reducing or eliminating overloaddistortion of signal processing circuits like microphone preamplifiersat high sound pressure levels. The overload and distortion problems areoften caused by saturation and non-linearity of active amplificationelements like transistors of the signal processing circuits.

SUMMARY

A first aspect relates to a microphone assembly including a transducerconfigured to convert sound into a microphone signal at a transduceroutput and a processing circuit. The processing circuit includes apreamplifier including an input node connected to the transducer outputfor receipt of the microphone signal, the preamplifier being configuredto generate at least one of an amplified microphone signal and abuffered microphone signal, and an analog-to-digital converter (ADC)configured to receive, sample and quantize the amplified or bufferedmicrophone signal to generate a corresponding digital microphone signal.The processing circuit additionally includes a feedback path including adigital loop filter which includes an adjustable or fixed transferfunction, the digital loop filter being configured to receive and filterthe digital microphone signal to provide a first digital feedback signaland a digital-to-analog converter (DAC) configured to convert the firstdigital feedback signal into a corresponding analog feedback signal. Theprocessing circuit additionally includes a summing node at thetransducer output configured to combine the microphone signal and theanalog feedback signal.

A second aspect relates to a method of controlling a frequency responseof a signal amplification path of a microphone. The method includes a)converting incoming sound into a corresponding microphone signal at amicrophone transducer output; b) sampling and quantizing the microphonesignal by an analog-to-digital converter (ADC) to generate acorresponding digital microphone signal; c) lowpass filtering thedigital microphone signal to provide a first digital feedback signal; d)converting the first digital feedback signal into a corresponding analogfeedback signal by digital-to-analog converter (DAC); and e) combiningthe microphone signal and the analog feedback signal at the microphonetransducer output to close a digital feedback loop.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the following drawings and thedetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present disclosure will becomemore fully apparent from the following description and appended claims,taken in conjunction with the accompanying drawings. These drawingsdepict only several embodiments in accordance with the disclosure andare, therefore, not to be considered limiting of its scope. Variousembodiments are described in more detail below in connection with theappended drawings.

FIG. 1 is a schematic illustration of an exemplary miniature microphoneassembly according to various embodiments.

FIG. 2 is a simplified electrical block diagram of a processing circuitof a miniature microphone assembly according to various embodiments.

FIG. 3 is a block diagram of a digital loop filter of a feedback loop orpath according to various embodiments.

FIG. 4 is a simplified block diagram of a hybrid Pulse-Width andPulse-Amplitude Modulator (PWAM) of a feedback loop or path of aprocessing circuit according to various embodiments.

FIG. 5 is a simplified block diagram of an exemplary embodiment of anoise-shaping up-sampler and quantizer.

FIG. 6 is a schematic block diagram of the operation of a modulatorportion of a hybrid Pulse-Width and Pulse-Amplitude Modulator (PWAM)according to various embodiments.

FIG. 7 is a simplified schematic block diagram of an exemplaryembodiment of a current output converter forming part of a current modeDAC of the processing circuit.

FIG. 8 is a simplified schematic block diagram of a controllable currentgenerator and the exemplary current output converter operating in anidle state according to various embodiments.

FIGS. 9A and 9B are simplified schematic block diagrams of acontrollable current generator operating in first and second states,respectively, according to various embodiments.

FIG. 10 is a simplified schematic block diagram of an exemplarymicrophone preamplifier of a processing circuit.

In the following detailed description, various embodiments are describedwith reference to the appended drawings. The skilled person willunderstand that the accompanying drawings are schematic and simplifiedfor clarity and therefore merely show details which are essential to theunderstanding of the present disclosure, while other details have beenleft out. Like reference numerals refer to like elements or componentsthroughout. Like elements or components will therefore not necessarilybe described in detail with respect to each figure. It will further beappreciated that certain actions and/or steps may be described ordepicted in a particular order of occurrence while those skilled in theart will understand that such specificity with respect to sequence isnot actually required.

DETAILED DESCRIPTION

One aspect of the present disclosure relates to a microphone assemblyincluding a transducer and a processing circuit. The processing circuitincludes an analog-to-digital converter (ADC) configured to receive,sample and quantize a microphone signal generated by the transducer togenerate a corresponding digital microphone signal. The processingcircuit includes a feedback path including a digital loop filterconfigured to receiving and filtering the digital microphone signal toprovide a first digital feedback signal and a digital-to-analogconverter (DAC) configured for converting the first digital feedbacksignal into a corresponding analog feedback signal. The processingcircuit additionally includes a summing node at the transducer outputconfigured to combining the microphone signal and the analog feedbacksignal.

Another aspect relates to a microphone assembly including a transducerconfigured to convert sound into a microphone signal at a transduceroutput and a processing circuit. The processing circuit includes apreamplifier including an input node connected to the transducer outputfor receipt of the microphone signal, the preamplifier being configuredto generate at least one of an amplified microphone signal and abuffered microphone signal, and an analog-to-digital converter (ADC)configured to receive, sample and quantize the amplified or bufferedmicrophone signal to generate a corresponding digital microphone signal.The processing circuit additionally includes a feedback path including adigital loop filter which includes an adjustable or fixed transferfunction, the digital loop filter being configured to receive and filterthe digital microphone signal to provide a first digital feedback signaland a digital-to-analog converter (DAC) configured to convert the firstdigital feedback signal into a corresponding analog feedback signal. Theprocessing circuit additionally includes a summing node at thetransducer output configured to combine the microphone signal and theanalog feedback signal.

The transducer may include a capacitive transducer (e.g., amicroelectromechanical system (MEMS) transducer) configured to convertincoming sound into a corresponding microphone signal. The capacitivetransducer may for example exhibit a transducer capacitance between 0.5pF and 10 pF. Some embodiments of the capacitive transducer may includefirst and second mutually charged transducer plates, e.g. a diaphragmand back plate, respectively, supplying the microphone signal. Thecharge may be injected onto one of the diaphragm and back plate by anappropriate high-impedance DC bias voltage supply. The processingcircuit may include a semiconductor die, for example a mixed-signal CMOSsemiconductor device integrating the preamplifier, analog-to-digitalconverter, digital loop filter, digital-to-analog converter andoptionally various other analog and digital circuits as discussed below.

The microphone assembly may be shaped and sized to fit into portableaudio and communication devices such as smartphones, tablets and mobilephones, etc. The transducer may be responsive to audible sound.

The analog-to-digital converter may be adapted to producing a multibitor single-bit digital microphone signal representative of the microphonesignal depending on the particular converter type. Some embodiments ofthe analog-to-digital converter may include an oversampled convertertype such as a single-bit or multibit sigma-delta converter (ΣΔ)configured to generate a single-bit (PDM) or multibit digital microphonesignal at a first sampling frequency. The multibit sigma-delta converter(ΣΔ) may be configured to generate the multibit digital microphonesignal with samples of two, three or four bits. The first samplingfrequency may lie between 1 MHz and 20 MHz, such as between 2.048 MHzand 4.196 MHZ, for example 3.092 MHz. The feedback path may include adecimator arranged in-front of an input of the digital loop filter. Thedecimator is configured to convert the single-bit (PDM) or multibitdigital microphone signal into a decimated multibit (PCM) microphonesignal at a second sampling frequency. The second sampling frequency islower than the previously discussed first sampling frequency. The secondsampling frequency may be between 8 and 64 times lower than the firstsampling frequency, e.g., accomplished by configuring the decimator withdecimation factors between 8 and 64, such as 16 or 32. The samples ofthe decimated multibit (PCM) microphone signal may possess a highernumber of bits, e.g., between 12 and 32 bits, or for example, 24 bits,than the samples of the single-bit or multibit digital microphone signalto retain high signal resolution through the feedback path despite thereduced sampling frequency. The skilled person will understand that thepreamplifier or buffer may be integrated within the analog-to-digitalconverter in some embodiments.

The digital-to-analog converter is configured to convert the firstdigital feedback signal into a corresponding analog feedback signalwhich is combined with the microphone signal at the transducer outputsuch that a feedback loop is closed in-front of and around themicrophone preamplifier. In some embodiments, the analog feedback signaland microphone signal may be summed by electrically connecting thetransducer output and an output of the digital-to-analog converter. Theapplication of the analog feedback signal to the transducer output ofthe transducer leads to numerous advantages compared with prior artapproaches. The coupling scheme effectively prevents low-frequencyoverload of the preamplifier or buffer which must receive and processthe full dynamic range of the microphone signal generated by thetransducer without unacceptable distortion. This is accomplished by ananti-phase low-frequency component supplied by the analog feedbacksignal which cancels or suppresses low-frequency components of themicrophone signal at the input node, or nodes, of the preamplifier orbuffer. Hence, attenuating or supressing high level low-frequencycomponents of the microphone signal at the input of the preamplifier orbuffer. The high level low-frequency components of the microphone signalare caused by the exposure to intense subsonic or low-frequency soundsgenerated by wind noise, large machinery, etc. This suppression of highlevel low-frequency components of the microphone signal at the input ofthe preamplifier or buffer markedly reduces the maximum signal level ofthe microphone signal which the preamplifier or buffer must be able tohandle in an undistorted manner. Hence, eliminating the vulnerability ofthe preamplifier or buffer to low-frequency induced overload anddistortion.

The preamplifier may be a DC-coupled design or an AC-coupled design. TheDC-coupled preamplifier or buffer possesses several advantageousproperties over the AC-coupled design due to the elimination ofresistors and capacitor of a traditional analog highpass filter used toset a highpass cut-off frequency of a forward microphone amplificationpath. The forward microphone amplification path may be extending fromthe transducer output to at least the analog-to-digital converteroutput. The elimination of the resistors and capacitors of the analoghighpass filter at the preamplifier or buffer leads to a smallersemiconductor die area, reduced thermal noise and much improvedflexibility in the choice of the highpass cut-off frequency of theforward microphone amplification path. Instead, the highpass cut-offfrequency of the forward microphone amplification path may be controlledor dominated by a lowpass cut-off frequency of the digital loop filteras discussed below. The frequency response of the digital loop filter,including its lowpass cut-off frequency, is inherently significantlymore accurate than the frequency response of the traditional analoghighpass filter, because resistors and capacitors of the analog highpassfilter exhibit substantial manufacturing spread and drift over time andtemperature making accurate frequency response control difficult,expensive or both.

The accurate frequency response setting of the forward microphoneamplification path afforded by the properties of the digital loop filteralso improves frequency response matching, inclusive phase matching,between individual microphone assemblies of a beamforming microphonearray. This improved response matching leads to improved, predictableand stable directional response of the beamforming microphone array.

The digital-to-analog converter may exhibit a very large outputimpedance to supply the current of the analog feedback signal into aload including a high-impedance capacitive transducer without causingundesirable attenuation and/or distortion of the microphone signal atthe transducer output. In certain embodiments, the output impedance ofthe digital-to-analog converter 10 kHz may be larger than 1 MΩ, such aslarger than 10 MΩ, or 100 MΩ.

The skilled person will understand that the analog feedback signal maybe directly connected to the transducer output, e.g., at least onetransducer plate of a capacitive transducer element. In that context,directly means through an electrically conductive path without anyintervening active devices like transistors, but possibly throughpassive components like resistors, capacitors, electrical traces, wires,etc. This feature effectively prevents the above-mentioned overload anddistortion problems of the preamplifier and buffer at high levels of themicrophone signal caused by saturation and non-linearity of activeamplification elements like transistors of the preamplifier or buffercircuitry. This low-frequency cut-off frequency may in practice beaccurately controlled by a setting of the lowpass cut-off frequency ofthe digital loop filter as discussed above. The digital signalprocessing of the digital loop filter allows a very accurate and stablesetting of the frequency response of the forward microphoneamplification path, in particular in combination with the DC-coupledpreamplifier or buffer as discussed above.

The digital loop filter may include a lowpass filter possessing acut-off frequency placed at or above 10 Hz, for example at or above 50Hz, 100 Hz or 1000 Hz. The lowpass filter may include a first, second orthird order response characteristic. The skilled person will understandthat the lowpass cut-off frequency of the digital loop filter may beselected such that a desired highpass cut-off frequency of the forwardmicrophone amplification path is obtained. The latter highpass cut-offfrequency may be situated between 10 Hz and 4000 Hz, such as between 100Hz and 1 kHz, for various embodiments of the microphone assemblydepending on requirements of a specific application. The digital loopfilter may include an adjustable or programmable transfer function incertain embodiments of the processing circuit. The transfer function maybe controlled by filter configuration data which may determine thepreviously discussed cut-off frequency of the lowpass filter. The filterconfiguration data may include respective values of one or more filtercoefficients of the digital loop filter. The filter configuration datamay be received by the processing circuit via an integrated command andcontrol interface. The integrated command and control interface allowsthe microphone assembly to connect to a compatible data interface of thehost processor and thereby receive the filter configuration data fromthe host processor. The programmable transfer function of the digitalloop filter allows the microphone assembly to be tailored torequirements of a particular application in connection with, or after,manufacturing in a flexible manner and therefore serves to reduce thenumber of variants needed of the microphone assembly.

The processing circuit may include a digital processor implementing thefunctionality of the digital loop filter and/or other control functionsof the processing circuit such as state switching of thedigital-to-analog converter, controlling the operation of a command andcontrol interface connectable to host processor of a portablecommunication device, e.g. a smartphone, etc. The digital processor maycomprise a digital state machine and/or a software programmablemicroprocessor such as a digital signal processor (DSP).

According to some embodiments, the digital-to-analog converter (DAC)includes a hybrid Pulse-Width and Pulse-Amplitude Modulator (PWAM)configured to generate the analog feedback signal by converting thefirst digital feedback signal into a corresponding pulse-width andpulse-amplitude modulated signal at a higher sampling frequency than asampling frequency of the first digital feedback signal. The pulse-widthand pulse-amplitude modulated signal may include a sequence of variablewidth and amplitude current pulses generated by a current outputconverter representative of the samples of the first digital feedbacksignal. Hence, one embodiment of the hybrid Pulse-Width andPulse-Amplitude Modulator includes a current output converter configuredfor converting the pulse-width and pulse-amplitude modulated signal intoa corresponding sequence of variable width and amplitude current pulses.The current output converter includes a plurality of individuallycontrollable current generators connected in parallel to the DAC output.The current output converter may include between 8 and 32 individuallycontrollable current generators connected in parallel to the DAC output.The skilled person will understand that the capacitance of thecapacitive transducer at the transducer output effectively lowpassfilters or smoothes the variable width and amplitude current pulses thatmay be supplied by the output of the hybrid Pulse-Width andPulse-Amplitude Modulator to suppress or eliminate undesired highfrequency components in the analog feedback signal. In theabove-mentioned current output converter each of the plurality ofindividually controllable current generators may include a first currentsource connected between a positive DC supply rail of the current outputconverter and the DAC output for sourcing a first current level to theDAC output; and a second current source connected between the DAC outputand a negative DC supply rail of the current output converter forsinking a second current level from the DAC output. The individuallycontrollable current generator may furthermore include a DC errorsuppression circuit configured for matching the first and second currentlevels. The matching or equalization of the first and second currentlevels by the operation of the DC error suppression circuit has severalnoticeable advantages, for example, leading to a linear I/Ocharacteristic of the current output converter. The DC error suppressioncircuit also prevents the build-up of DC voltage components on the loadwhich is a noticeable advantage in connection with driving capacitivetransducer elements where DC off-sets or DC imbalances of the outputsignal at the DAC output will tend to drive a DC operating point of thecapacitive transducer away from a target DC operating point as discussedin further detail below with reference to the appended drawings.

Some embodiments of the hybrid Pulse-Width-Modulator andPulse-Amplitude-Modulator may include a noise-shaping quantizerconfigured to receive samples, having a first bit-width, of the firstdigital feedback signal outputted by the digital loop filter; andquantize the samples of the first digital feedback signal to generatesamples of a second digital feedback signal with a reduced bit-width.The noise-shaping quantizer may quantize samples of an incoming digitalsignal to fewer bits, e.g., samples with reduced bit-width. Thenoise-shaping quantizer may for example quantize samples of the firstdigital feedback signal from e.g., 32 bits or 24 bits down to less than16 bits, or less than 12 bits, such as 11 bits. The noise-shapingquantizer is configured to shape a spectrum of the quantization noisegenerated by the quantization process to reduce its audibility. Hence,the quantization noise may be pushed upwards in frequency above theaudible range, for example above 20 kHz. The first digital feedbacksignal may have a sampling frequency at or above 48 kHz, for exampleabove 96 kHz, such as 192 kHz or 384 kHz. The second digital feedbacksignal may subsequently be converted into the pulse-width andpulse-amplitude modulated signal as discussed in further detail belowwith reference to the appended drawings.

The hybrid Pulse-Width and Pulse-Amplitude Modulator is capable ofgenerating the analog feedback signal with a high resolution at arelatively low conversion frequency as discussed in further detail belowwith reference to the appended drawings. The skilled person willunderstand that alternative embodiments of the hybrid Pulse-Width andPulse-Amplitude Modulator may lack the current output converter andinstead exhibit a relatively small output impedance supplying thepulse-width and pulse-amplitude modulated signal as a sequence ofvariable width and amplitude voltage pulses. The hybrid Pulse-Width andPulse-Amplitude Modulator disclosed herein may be exploited forconversion of digital signals into corresponding analog signals in avariety of other applications than the microphone assembly describedherein.

The skilled person will also understand that the current outputconverter described herein can be utilized for DC error-free conversionof a digital signal to an analog signal in numerous other applicationsthan the hybrid Pulse-Width and Pulse-Amplitude Modulator describedherein.

Further aspects of the present disclosure relate to a method ofcontrolling a frequency response of a signal amplification path of amicrophone. The method includes a) converting incoming sound into acorresponding microphone signal at a microphone transducer output; b)sampling and quantizing the microphone signal by an analog-to-digitalconverter (ADC) to generate a corresponding digital microphone signal;c) lowpass filtering the digital microphone signal to provide a firstdigital feedback signal; d) converting the first digital feedback signalinto a corresponding analog feedback signal by digital-to-analogconverter (DAC); and e) combining the microphone signal and the analogfeedback signal at the microphone transducer output to close a digitalfeedback loop.

The methodology may further include 0 applying the analog feedbacksignal to a capacitive transducer for example applying the analogfeedback signal to at least one transducer plate of a capacitivemicroelectromechanical (MEMS) transducer supplying the microphonesignal. The application of the analog feedback signal of the capacitivetransducer allows accurate control of the frequency response of theforward microphone amplification path for the reasons discussed inconsiderable detail above.

The method of controlling the frequency response of the signalamplification path may further include g) quantizing and noise shapingthe first digital feedback signal to generate a second digital feedbacksignal with a smaller bit-width than a bit-width of the first digitalfeedback signal.

The bit-width of the first digital feedback signal may be larger than 20bits, such as 24 bits, while the bit-width of the second digitalfeedback signal may be less than 16 bits, such as 11 bits, e.g., 10 bitsmagnitude representation and one sign bit, as discussed in furtherdetail below with reference to the appended drawings.

The method of controlling the frequency response of the signalamplification path may further include increasing a sampling frequencyof the second digital feedback signal with a predetermined upsamplingfactor, N, to generate a third digital feedback signal; and convertingthe third digital feedback signal into a pulse-width and pulse-amplitudemodulated signal.

The skilled person will understand that the above conversion of thethird digital feedback signal may include h) dividing a first sample ofthe third digital feedback signal with the predetermined upsamplingfactor, N, to compute a modulus value and remainder value of the firstsample; i) converting the modulus value of the first sample into a firstpulse segment having a combined pulse width and pulse amplituderepresentative of the modulus value; j) converting each remainder valueof the first sample into a second pulse segment with a pulse widthrepresenting the remainder value; k) combining the first and secondpulse segments of the first sample to form a first variable width andamplitude pulse; and l) repeating steps h)-k) for subsequent samples ofthe third digital feedback signal to generate the pulse-width andpulse-amplitude modulated signal by a sequence of variable width andamplitude pulses. Hence, the sequence of variable width and amplitudepulses are representative of the samples of the third digital feedbacksignal as discussed in further detail below with reference to theappended drawings.

Further aspects of the present disclosure relate to a semiconductor dieincluding a processing circuit according to any of the above-describedembodiments thereof. The processing circuit may include a CMOSsemiconductor die. The processing circuit may be shaped and sized forintegration into a miniature microphone housing or package. Themicrophone assembly may therefore include a microphone housing enclosingand supporting the transducer and the processing circuit. A bottomportion of the microphone housing may include a carrier board, such as aprinted circuit board, onto which the processing circuit and thetransducer are attached or fixed by a suitable bonding mechanism. Themicrophone housing may include a sound port or inlet allowing soundpassage to the transducer as discussed in further detail below withreference to the appended drawings.

Further aspects of the present disclosure relate to a portablecommunication device including a microphone assembly according to any ofthe above-described embodiments thereof. The portable communicationdevice may include an application processor, e.g., a microprocessor suchas a Digital Signal Processor. The application processor may include adata communication interface compliant with, and connected to, anexternally accessible data communication interface of the microphoneassembly. The data communication interface may include a proprietaryinterface or a standardized data interface, such as one of I²C, USB,UART, SoundWire or SPI compliant data communication interfaces. Varioustypes of configuration data of the processing circuit, for example forprogramming or adapting characteristics of the digital loop filter, maybe transmitted from the application processor to the microphoneassembly, as discussed in further detail below with reference to theappended drawings.

In some embodiments, the present microphone assembly may form part of aportable communication device such as a smartphone where one, two, threeor more microphone assemblies may be integrated for picking-up andprocessing various types of acoustic signals such as speech and music.Some exemplary embodiments of the present approaches, microphoneassemblies and methodologies may be tuned or adapted to different typesof applications through configurable parameters as discussed in furtherdetail below. These parameters may be loaded into suitable memory cellsof the microphone assembly on request via the configuration datadiscussed above, for example, using the previously mentioned command andcontrol interface. The latter may include a standardized datacommunication interface such as SoundWire, I2C, UART and SPI.

FIG. 1 shows an exemplary embodiment of a microphone assembly or system100. The microphone assembly 100 includes a capacitive transducer 102,e.g. a microelectromechanical system (MEMS) transducer, configured toconvert incoming sound into a corresponding microphone signal. Thecapacitive transducer includes a diaphragm 105 movable relative to aback plate 106 in response to changes in air pressure entering thehousing via the sound port 109. The diaphragm 105 includes a pressureequalization opening or pierce 113 that inherently functions as anacoustic high pass filter with a cutoff frequency that is a function ofthe diameter of the opening. The transducer 102 may, for example,exhibit a transducer capacitance between 0.5 pF and 10 pF. Thecapacitive transducer may include first and second mutually chargedtransducer plates, e.g. a diaphragm and back plate, respectively,supplying the microphone signal. The charge may be injected onto one ofthe diaphragm and back plates by an appropriate high-impedance DC biasvoltage supply (not shown). The microphone assembly 100 additionallyincludes a processing circuit 122, which may include a semiconductordie, for example a mixed-signal CMOS semiconductor device integratingthe various analog and digital circuits discussed below. The processingcircuit 122 is shaped and sized for mounting on a substrate or carrierelement 111 of the assembly 100, where the carrier element likewisesupports the capacitive transducer 102. The microphone assembly 100includes a housing lid 103 mounted onto a peripheral edge of thesubstrate or carrier element 111 such that the housing lid 103 andcarrier element 111 jointly form a microphone housing 110 enclosing andprotecting the transducer 102 and the processing circuit 122 of theassembly 100. The microphone housing 110 may include a sound inlet orsound port 109 projecting through the carrier element 111, or throughthe housing lid 103 in other embodiments, for conveying sound waves tothe transducer 102.

The transducer 102 generates a microphone signal at a transducer output(item 101 a of FIG. 2) in response to impinging sound. The transduceroutput may for example include a pad or terminal that is electricallycoupled to the processing circuit 122 via one or more bonding wires 107electrically interconnecting respective signal pads of the transducer102 and processing circuit 122.

FIG. 2 shows a simplified electrical block diagram of an exemplaryembodiment of the processing circuit 122 of the miniature microphoneassembly 100. The processing circuit 122 includes a preamplifier orbuffer 104 having an input node or terminal 101 b connected to thetransducer output 101 a of the transducer 102 for receipt of themicrophone signal produced by the transducer 102. The output of thepreamplifier 104 supplies an amplified and/or buffered microphone signalto an analog-to-digital converter 108, which is configured for receipt,sampling and quantization of the amplified or buffered microphone signalto generate a corresponding digital microphone signal. Theanalog-to-digital converter 108 may be adapted to produce a multibit orsingle-bit digital microphone signal representative of the microphonesignal depending on the particular converter type. Some embodiments ofthe analog-to-digital converter 108 includes a sigma-delta converter(ΣΔ) configured to generate a single-bit (PDM) digital microphone signalat a first sampling frequency. The first sampling frequency may liebetween 2 MHz and 20 MHz such as 3.092 MHz. The skilled person willunderstand that the preamplifier 104 may be integrated with theanalog-to-digital converter 108 in other embodiments.

The digital microphone signal is transmitted to an input of a commandand control interface 114 configured to receive various types of datacommands and filter configuration data for a programmable digital loopfilter 120 from a host processor (now shown) of a portable communicationdevice (e.g. a smartphone). The command/control interface 114 mayinclude a separate clock line 116 (CLK) that clocks data on a data line118 (DATA) of the interface 114. The command and control interface 114may include a standardized data communication interface according tovarious serial data communication protocols, e.g., I²C, USB, UART,SoundWire or SPI. The command and control interface 114 is configured tostructure and encode the digital microphone signal in accordance withthe relevant protocol of the interface 114 and transmits the digitalmicrophone signal to the host processor. The microphone assembly 100 maybe configured to receive and utilize various types of configuration datatransmitted by the host processor. The configuration data may includedata concerning a configuration of the processing circuit 122 such asfilter coefficients of the digital loop filter 120.

The processing circuit 122 includes a feedback path extending at leastfrom the digital microphone signal at the output 112 of theanalog-to-digital converter 108 and back to the transducer output 101 a,or microphone preamplifier input node 101 b, since these nodes areelectrically connected. The feedback path supplies an analog feedbacksignal to the summing node 132 at the transducer output 101 a such thatthe path may be operative to set a highpass cut-off frequency of thefrequency response of the forward microphone amplification path from thetransducer output to the output 112 of the analog-to-digital converter108. This highpass cut-off frequency may in practice be accuratelycontrolled by a setting of a lowpass cut-off frequency of the digitalloop filter 120 discussed above. The digital loop filter may via itsdigital processing nature exhibit a very accurate and stable frequencyresponse setting in contrast to frequency response settings ofconventional analog filters which rely on values of components likecapacitors and resistors to determine the frequency response. Componentslike capacitors and resistors exhibit substantial manufacturing spreadof component values and drift over time and temperature such that thefrequency response setting of conventional analog filters are lessaccurate and stable than desired.

If the analog-to-digital converter 108 produces a digital microphonesignal in the form of a single-bit (PDM) digital microphone signal or amultibit digital microphone signal with 2-4 bits, the digital feedbackloop of the processing circuit 122 may include a decimator 115 arrangedin-front of, e.g., at the input of, the digital loop filter 120. Thisdecimator 115 is configured to convert the single-bit (PDM) or multibitdigital microphone signal into a decimated multibit (PCM) feedbacksignal at a second sampling frequency. The second sampling frequency islower than the first sampling frequency, which may lie between 2 MHz and20 MHz, of the single-bit (PDM) digital microphone signal as discussedabove. The second sampling frequency may be between 8 and 64 times lowerthan the first sampling frequency, e.g. accomplished by configuring thedecimator 115 with decimation factors between 8 and 64, such as 16 or32. The samples of the decimated multibit feedback signal may includebetween 16 and 32 bits to maintain a high signal resolution in thedigital feedback path. This decimation and associated lowpass filteringof the single-bit (PDM) digital microphone signal may be helpful tosuppress high-frequency noise components of the single-bit (PDM) digitalmicrophone signal. The decimated multibit feedback signal is applied toan input of the digital loop filter 120 which filters the signal inaccordance with an adjustable or fixed transfer function, such as thepreviously discussed lowpass frequency response, of the filter 120 to inresponse generate a first digital feedback signal at a filter output.Exemplary topologies and transfer functions of the digital loop filter120 are discussed in detail below.

The first digital feedback signal supplied by the digital loop filter120 is applied to a digital-to-analog converter (DAC) of the digitalfeedback loop. The DAC includes a hybrid Pulse-Width and Pulse-AmplitudeModulator (PWAM) 125 connected in series with a current output converter(IDAC) 130. The DAC is configured to convert the first digital feedbacksignal into a corresponding analog feedback signal, which is applied tothe microphone preamplifier input node 101 b. The latter node 101 b isconnected to the transducer output 101 a as discussed herein, therebyclosing the digital feedback loop. The skilled person will understandthat the transducer output may be an extremely high impedance circuitnode, e.g. an impedance corresponding to a capacitance of 0.5 pF to 10pF, of a miniature capacitive transducer element. This property of thetransducer output, and the design and electrical properties, inparticular output impedance, of the current output converter 130 arediscussed in further detail below with reference to the schematicdiagram of the converter 130. The application of the analog feedbacksignal to the transducer output of the miniature capacitive transducer102 leads to numerous advantages compared with prior art approaches. Thedirect coupling of the analog feedback signal to the transducer outputeffectively prevents low-frequency overload of the preamplifier orbuffer 104 and/or the converter 108. This is accomplished by the lowpassfiltering of the analog feedback signal carried out by the digital loopfilter 120, which lowpass filtering cancels or suppresses low-frequencycomponents of the microphone signal at the input node 101 b, or inputnodes, of the preamplifier or buffer 104. Furthermore, the noise floorof the microphone assembly 100 may be lowered by tailoring a frequencyresponse of the miniature capacitive transducer 102 to the accuratefrequency response of the forward microphone amplification path.Furthermore, the accurate control over the frequency response of theforward microphone amplification path improves frequency responsematching, including phase matching, between individual microphoneassemblies of a beamforming microphone array which may include 2, 3 ormore microphone assemblies. This improved response matching leads toimproved, predictable and stable directional response of the beamformingmicrophone array.

The upper portion of FIG. 3 shows a block diagram of an exemplaryembodiment of the previously discussed digital loop filter 120 of thedigital feedback loop or path of the processing circuit 122. The digitalloop filter 120 has second order lowpass filter characteristics using aclassical IIR filter bi-quad topology. The skilled person willunderstand that other digital filter types and topologies, such as FIRfilters or other types of IIR filter topologies, may be utilized inalternative embodiments of the digital loop filter 120. Likewise, otherfilter orders may be used. The transfer function of the illustrateddigital loop filter 120 is determined by values of the filtercoefficients which include: a1, a2, b0, b1 and b2. The frequencyresponse graph 300 of the lower portion of FIG. 3 shows an exemplarymagnitude response 310 of the digital loop filter 120 where the lowpasscut-off frequency has been tuned to about 200 Hz. The correspondingmagnitude response 305 of the forward microphone amplification path forthis particular setting of the response 310 of the digital loop filteris also plotted. The skilled person will notice the expected 2^(nd)order highpass magnitude response of the forward microphoneamplification path. The highpass cut-off frequency is set toapproximately 30 Hz. The skilled person will understand that the lowpasscut-off frequency of the digital loop filter 120 may be adjusted over abroad frequency range to obtain a desired highpass cut-off frequency ofthe forward microphone amplification path. The latter highpass cut-offfrequency may be situated in the frequency range between 10 Hz and 200Hz for various embodiments of the microphone assembly depending onrequirements of a specific application.

The skilled person will understand that certain embodiments of theprocessing circuit 122 may include an adjustable or programmabletransfer function of the digital loop filter 120 where the transferfunction is controlled by filter configuration data. The filterconfiguration data may include respective values of one or more of thepreviously discussed filter coefficients a1, a2, b0, b1 and b2. Thefilter configuration data may be received by the processing circuit 122via the previously discussed command and control interface 114 from ahost processor. The programmable transfer function of the digital loopfilter 120 allows the microphone assembly to be tailored to requirementsof a particular application after manufacturing in a flexible manner andtherefore reduces the number of variants needed of the microphoneassembly.

Other types of configuration data for various circuits and functions ofthe processing circuit 122 may likewise be programmed through thecommand and control interface 114. The configuration data, includingfilter configuration data, may be stored in rewriteable memory cells(not shown) of the processing circuit, such as flash memory, EEPROM,RAM, register files or flip-flops. These rewriteable memory cells mayhold or store certain default values of the filter configuration data.

FIG. 4 shows a simplified block diagram of an exemplary embodiment ofthe previously discussed hybrid Pulse-Width and Pulse-AmplitudeModulator (PWAM) 125 of the feedback path of the processing circuit. Theoutput of the digital loop filter 120 is connected to the input of thePWAM 125 such that the previously discussed first digital feedbacksignal is applied to the input of the PWAM 125. The first digitalfeedback signal may be a multibit signal with a relatively highresolution—for example between 16 and 32 bits per sample, such as 24bits per sample, to maintain a high signal resolution through thefeedback path. The sampling frequency of the first digital feedbacksignal may lie between 32 kHz and 384 kHz, for example between 96 kHzand 192 kHz. The PWAM 125 includes a noise-shaping up-sampler andquantizer 410 at the input receiving the first digital feedback signal.The noise-shaping up-sampler and quantizer 410 raises the samplingfrequency of the first digital feedback signal with a pre-set orprogrammable ratio—for example an integer ratio between 2 and 16 togenerate a second digital feedback signal 415 at a second samplingfrequency. The noise-shaping up-sampler and quantizer 410 is furthermoreconfigured to quantize samples of the second digital feedback signal toa smaller number of bits than the samples of the first digital feedbacksignal. According to one exemplary embodiment of the quantizer 410 thesamples of the first digital feedback signal has 24 bits per samplewhile the samples of the second digital feedback signal have beendecimated to 11 bits. These samples may be generated according to asigned sample format where a sign bit takes one bit and a magnitudeportion is represented by the residual 10 bits of the sample.

FIG. 5 shows a simplified block diagram of the noise-shaping up-samplerand quantizer 410 of the PWAM 125. The first digital feedback signal isrepresented by X(z) and the second digital feedback signal by X(z)+E(z),where E(z) represents a quantization noise component caused by thequantization operation carried out by the quantizer 504. Thenoise-shaping up-sampler and quantizer 410 includes a noise-shapingfeedback loop extending through loop filter Hn(z) 506 to a second adder510 on the input side, which adder shapes the spectrum of the generatedquantization noise to higher frequencies and therefore maintains arelatively high resolution of the second digital feedback signalthroughout the audio frequency range despite the quantization. Thenoise-shaping up-sampler and quantizer 410 may include a feedforwardloop as illustrated extending to an output side summer 512. Thenoise-shaping up-sampler and quantizer 410 may also include theillustrated Dither generator 502, which adds a pseudo-random noisesignal of appropriate level to the first digital feedback signal at theinput of the noise-shaping up-sampler and quantizer 410 using a firstinput side adder 508. This pseudo-random noise signal may reduce audibleartefacts associated with the quantization operation in a well-knownmanner.

Referring further to FIG. 4, the PWAM 125 additionally includes amodulator 420 connected to the output of the noise-shaping up-samplerand quantizer 410 for receipt of the second digital feedback signalX(z)+E(z). The operation and functionality of an exemplary embodiment ofthe modulator 420 is schematically illustrated in FIG. 6. The modulator420 takes the second feedback digital signal in the multibit (PCM)format and converts the second feedback digital signal into apulse-width and pulse-amplitude modulated signal. The sampling frequencyof this pulse-width and pulse-amplitude modulated signal may be markedlyhigher than the sampling frequency of the second digital feedback signalas discussed below. The sampling frequency of the pulse-width andpulse-amplitude modulated signal 425 may be at least 16 times higherthan the sampling frequency of the second digital feedback signal, suchas 32 or 64 times higher. One embodiment of the modulator 420 accepts a192 kHz sampling frequency of the second digital feedback signal andgenerates a corresponding pulse-width and pulse-amplitude modulatedsignal at a sampling frequency of 12.288 MHz, and hence raises thesampling frequency of the latter by an upsampling factor of 64. Thepulse-width and pulse-amplitude modulated signal 425 may be applied to acurrent converter control circuit or block 430—(see, e.g., FIG. 4). Thecurrent converter control circuit 430 is configured to convert ortransform the pulse-width and pulse-amplitude modulated signal into acorresponding sequence of variable width and amplitude current pulses atthe output of the previously discussed current output converter (IDAC)130 by controlling how individually controllable current generators(illustrated in FIGS. 7 and 8) are activated. The current convertercontrol circuit 430 may include an appropriately configured digitalstate machine. One embodiment of the current converter control circuit430 may include a dynamic element matching circuit 432 as schematicallyillustrated where the selection of individually controllable currentgenerators of the current output converter is carried out in arandomized manner to average out offsets between nominally identicalcurrent generators.

Referring to FIG. 6, the second digital feedback signal X(z)+E(z) isapplied to the input of the modulator 420 and the sampling frequencyraised with a predetermined ratio, N, such as 64. In the presentembodiment, the resolution of the second digital feedback signal is 11bits as discussed previously. A dividing block or circuit 603 divideseach 11 bits sample of the second digital feedback signal with N tocompute respective modulus values and remainder values of the samples.The drawing shows four exemplary values, 138, 40, 522 and 276 usingdecimal notation, of the 11 bit samples of the second digital feedbacksignal expressed in decimal format initially. The decimal sample value138 is divided by 64 producing a modulus value of 2 and a remaindervalue of 10 as illustrated. The corresponding computation is alsoillustrated for the three remaining samples 40, 522 and 276. The decimalsample value 138 is converted into binary format showing how the modulusvalue 2 corresponds to 00010b and the remainder value of 10 correspondsto 001010b. A first variable width and amplitude pulse 610 of thepulse-width and pulse-amplitude modulated signal is generated byconversion of the decimal sample value 138. The first variable width andamplitude pulse 610 is essentially constructed from two segments. Afirst pulse segment (2*64) has an amplitude of “2” (y-axis scale)spanning over a full pulse width, i.e. 100% modulation and pulseamplitude of 2—hence representing the modulus value “2.” The firstvariable width and amplitude pulse 610 additionally includes a secondpulse segment (1*10) spanning over merely 10 sample time clocks of the12.288 MHz sampling frequency of the pulse-width and pulse-amplitudemodulated signal. Stated in another way, the decimal sample value 138 isconverted into an “analog” variable width and amplitude pulse with acorresponding pulse area.

The conversion of the decimal sample value 40 into the second variablewidth and amplitude pulse 620 is also illustrated. The decimal samplevalue 40 leads to a modulus value of 0 and a remainder value of 40 asillustrated. The corresponding, second, variable width and amplitudepulse 620 reflects this outcome by merely including a second pulsesegment (1*40) with a “one” amplitude and spanning over merely 40 sampletime clocks of the 12.288 MHz sampling frequency of the pulse-width andpulse-amplitude modulated signal. The conversion of the decimal samplevalue 522 into a third variable width and amplitude pulse 630 is finallyillustrated using the same principles outlined above. The skilled personwill understand that the modulator 420 is configured to convert incomingsample values into corresponding sequences of variable width andamplitude pulses where the pulse area of each of the variable width andamplitude pulses 610, 620, 630 represents the sample value in question.Hence, each of the variable width and amplitude pulses 610, 620, 630 canbe viewed as an analog representation of the sample value in question.

The skilled person will understand that the modulator 420 may beconfigured to generate the variable width and amplitude pulses followingdifferent modulation schemes. In the present embodiment, each of thevariable width and amplitude pulses is preferably centred at a midpointof the pulse period, e.g., centered at the sample clock time 32 in thisembodiment using an upsampling factor of 64. This centering is oftenreferred to as a double-edge pulse-width modulation. However, otherembodiments of the modulator 420 may be adapted to build the variablewidth and amplitude pulses by applying single-edge modulation.

FIG. 7 shows a simplified schematic block diagram of an exemplaryembodiment of the current output converter 130 forming part of thecurrent mode DAC of the processing circuit. The current output converter130 includes a predetermined number, N, of individually controllablecurrent generators IDAC1, IDAC2, IDAC3, IDACN, for example between 4 and32 current generators, such as 16 current generators. The respectiveoutputs of the N individually controllable current generators areconnected in parallel to a common DAC output node 131. A capacitivetransducer 702 is connected to the common DAC output node 131. Theskilled person will understand that the capacitive transducer 702 mayinclude the previously discussed capacitive transducer 102 of aminiature microphone assembly for sound reproduction. However, othertypes of capacitive transducer elements for sensing of various types ofphysical variables may in the alternative be driven by the presentcurrent mode DAC 702. The N individually controllable current generatorsIDAC1, IDAC2, IDAC3, IDACN may be nominally identical, but the skilledperson will understand that component variations associated withsemiconductor manufacturing may cause minor variations ofcharacteristics between the controllable current generators, inparticular current sinking and sourcing capabilities. Each of the Nindividually controllable current generators IDAC1, IDAC2, IDAC3, IDACNis configured to selectively source current into the capacitivetransducer 702 or sink current from the capacitive transducer 702 inaccordance with the switching control carried out by the currentconverter control circuit 430, and thereby charge or discharge voltageacross transducer 702. Each of the N individually controllable currentgenerators IDAC1, IDAC2, IDAC3, IDACN can be considered a one-bit or 1.5bit binary values +1 or −1. The sourcing and sinking of thepredetermined current amount or level may be carried out by selectingbetween a first state and second state of the controllable currentgenerator. Finally, each of the individually controllable currentgenerators may include a third state or an idle/zero output state wherethe current generator neither sources nor sinks current to/from itsoutput. In this idle state, the current generator may be placed in ahigh-impedance mode effectively disconnecting the current generator fromthe common DAC output 131 as discussed in further detail below. Theskilled person will appreciate that the maximum positive output value ofthe current converter may correspond to setting all N individuallycontrollable current generators IDAC1, IDAC2, IDAC3, IDACN to sourcecurrent while the maximum negative output value corresponds to settingall N individually controllable current generators IDAC1, IDAC2, IDAC3,IDACN to sink current.

FIG. 8 is a simplified schematic block diagram of the controllablecurrent generator IDACN of the current output converter 130 when placedin the idle state or off-state discussed above. The controllable currentgenerator IDACN includes a first current source 802 and a second currentsource 804 connected in series between the positive DC supply rail VDDand a negative DC supply rail which is ground (GND) in the presentembodiment. A first switch pair, including switches SW2 and SW5, iscoupled in-between the first and second current sources 802, 804 and isoperating in a synchronized manner where both switches aresimultaneously closed/conducting or open/non-conducting. The switches ofthe first switch pair SW2 and SW5 are closed in the idle state while theresidual SW1, SW3, SW4 and SW6 are placed in open/non-conducting statesas illustrated. This means that the current flowing through the firstcurrent source 802 and second current source 804 runs directly from VDDto GND as illustrated by the current path 810. Consequently, each of thefirst and second current sources 802, 804 is electrically disconnectedfrom the output node 831 and the controllable current generator IDACNdoes therefore not source or sink any noticeable current to thecapacitive transducer 702 when placed in the idle state.

The controllable current generator IDACN additionally includes a DCvoltage reference 806 connected to an inverting input of a differentialloop amplifier 808, e.g. an operational amplifier or other differentialamplifier, of a feedback regulation loop of the IDACN. The voltage ofthe DC voltage reference 806 may be equal to one-half VDD. Thedifferential loop amplifier 808 has a non-inverting input (+) connectedto a midpoint node 812 arranged in-between the first switch pair SW2,SW5. An output of the differential loop amplifier 808 is connected to acontrol input 805 of the second current source 804 where the controlinput 805 is configured to adjust the current level of the secondcurrent source 804. The operation of the differential loop amplifier 808therefore seeks to dynamically or adaptively adjust the voltage at themidpoint node 812 to approximately one-half VDD, which is the voltageset at the negative input of the differential loop amplifier 808, byadjusting the current flowing through second current source 804 via thecontrol input 805. This adaptive adjustment of the voltage at themidpoint node 812 is carried out by a feedback regulation loop. Hence,the differential loop amplifier 808, the second current source 804 andthe DC voltage reference 806 therefore jointly form a DC errorsuppression circuit which is configured to match or align the first andsecond current levels supplied by the first and second current sources802, 804 during the idle state of the controllable current generatorIDACN. In certain embodiments, the differential loop amplifier 808 maypossess a relatively small bandwidth, or large time constant, comparedto the sampling frequency of the incoming pulse-width andpulse-amplitude modulated signal. The upper cut-off frequency of thedifferential loop amplifier 808 may, for example, be smaller than 100kHz, or smaller than 40 kHz, which effectively performs a slow averagingof the current source balancing to secure a long-term zero DC offset atthe output of each of the controllable current generators.

This property has several noticeable advantages, for example leading toa linear I/O characteristic of the current output converter 130. The DCerror suppression circuit also prevents build-up of DC voltagecomponents on the load, which is a noticeable advantage in connectionwith driving capacitive transducer elements where DC off-sets or DCimbalances of the analog feedback signal will tend to drive the DCoperating point of the capacitive transducer away from a target DCoperating point. This potential build-up of DC off-set is caused by thecharge integration carried out by the capacitance of the capacitivetransducer element. The controllable current generator IDACN isoperating in the previously discussed idle state where the output node831 is in a high-impedance state supplying substantially zero currentoutput. Each of the switches SW1, SW2, SW3, SW4, SW5 and SW6 may includea controllable semiconductor switch for example a MOSFET. Each of theswitches SW1, SW2, SW3, SW4, SW5 and SW6 may include a control terminal,for example a gate terminal of a MOSFET, which switches the controllablesemiconductor switch between its conducting and non-conducting states.These control terminals are connected to the previously discussedcurrent converter control circuit 430. The current level supplied by thefirst and second current sources 802, 804 may vary depending onrequirements of a particular application such as a load impedance, e.g.the capacitance of the capacitive transducer 702 in the presentembodiment, the sampling frequency of the pulse-width andpulse-amplitude modulated signal, the number of parallel connectedcontrollable current generators of the current output converter 130,etc. In one exemplary embodiment of the current output converter 130comprising 16 controllable current generators, the respective currentsof the first and second current sources 802, 804 are set to about 100pA, e.g. between 50 pA and 200 pA, when configured for driving a 1-4 pFcapacitive transducer element. The current settings of the controllablecurrent generators generally depend on a dv/dt at the peak amplitude ofthe analog feedback signal at the highest frequency of interest of thefeedback loop. The currents of the controllable current generatorsshould preferably be capable of charging the capacitance of thecapacitive transducer 102 without slew-induced distortion under theseconditions. The highest frequency of interest of the analog feedbacksignal may lie between 300 Hz and 3 kHz, for example about 1 kHz, inexemplary embodiments of the microphone assembly 100.

An output impedance at 10 kHz of each of the individually controllablecurrent generators IDAC1, IDAC2, IDAC3, IDACN is in some embodimentspreferably larger than 1 MΩ, such as larger than 10 MΩ or 100 MΩ, whenoperating in either the first state or the second state.

FIG. 9A is a simplified schematic block diagram of the controllablecurrent generator IDACN of the current output converter 130 when placedin the first state, or +1 state, discussed above where the output 831 issourcing the predetermined current level to the capacitive transducer702 or other load circuit. In the first state, the switches of the firstswitch pair SW2 and SW5 are open or non-conducting and the switches SW1and SW6 are both open or non-conducting as illustrated. The residualswitches SW4 and SW3 are in contrast placed in conducting or closedstates as illustrated. This combination of switch states means that thecurrent flowing through the first current source 802 is sourced into thecapacitive transducer 702 via current path 810 a while the currentgenerated by the second current source 804 runs from the DC voltagereference 806, which may be equal to one-half VDD, directly to GND viathe current path 810 b. Consequently, the controllable current generatorIDACN sources the predetermined current level to the capacitivetransducer 702 when placed in the first state. The skilled person willunderstand that the DC balancing of the current levels of the firstcurrent source 802 and the second current source 804 is still maintainedby the operation of the previously discussed DC error suppressioncircuit.

FIG. 9B is a simplified schematic block diagram of the controllablecurrent generator IDACN of the current output converter 130 when placedin the second state, or −1 state, discussed above where the output 831is sinking the predetermined current level from the capacitivetransducer 702 or other load circuit to discharge the load circuit. Inthe second state, the switches of the first switch pair SW2 and SW5 areopen or non-conducting and the switches SW4 and SW3 are both open ornon-conducting as illustrated. The residual switches SW1 and SW6 are incontrast placed in conducting or closed states as illustrated. Thiscombination of switch states means that the current flowing through thefirst current source 802 is sourced into the DC voltage reference 806and thereafter to GND via the current path 880 a. In contrast, thepredetermined current generated by the second current source 804 isdrawn out of the capacitive transducer 702 via current path 880 b todischarge the capacitive transducer 702. Consequently, the controllablecurrent generator IDACN sinks the predetermined current level from thecapacitive transducer 702 when placed in the second state. The skilledperson will understand that the DC balancing of the current levels ofthe first current source 802 and the second current source 804 is stillmaintained by the operation of the previously discussed DC errorsuppression circuit.

The skilled person will understand that above outlined switcharrangement and associated switching scheme of the switches SW1, SW2,SW3, SW4, SW5 and SW6 through the first, second and third states of eachof the controllable current generators allow the first and secondcurrent sources to operate in an unswitched manner even during timeperiods where they do not source or sink current to the load circuit.Instead, the superfluous current of a particular current generator isdirected through the DC voltage reference 806 by selecting anappropriate setting of the switches. This feature eliminates switchingnoise for example caused by charge injection from repetitious switchingof the first and second current sources when cycling through the first,second and third states.

FIG. 10 is a simplified schematic block diagram of an exemplaryAC-coupled microphone preamplifier 104 suitable for use in theprocessing circuit to interface to the transducer at the input side andto a differential input of the previously discussed analog-to-digitalconverter 108 at the output side. The skilled person will understandthat other embodiments of the processing circuit may include aDC-coupled preamplifier to achieve the previously discussed advantages.

The microphone preamplifier 104 includes a unity gain buffer stage 1001coupled in series with an AC-coupled gain stage 1003. The input voltageVin to the unity gain buffer stage 1001 includes the microphone signalsupplied via the previously discussed (See, e.g., FIG. 2) input node 101b connected to the transducer output 101 a of the transducer 102. Theunity gain buffer stage 1001 is single-ended in the present embodimentand delivers a buffered microphone signal or voltage to the input 1005of the AC-coupled gain stage 1003. The small signal gain of theAC-coupled gain stage 1003 to a positive output at Vop is determined bya capacitance ratio between capacitors C1 and C2 and may lie between 12and 30 dB. The small signal gain of the AC-coupled gain stage 1003 to anegative output at Von is determined by a capacitance ratio betweencapacitors C4 and C3. The latter small signal is preferably set equal tothe small signal gain to the positive output at Vop of the AC-coupledgain stage 1003. Consequently, the microphone preamplifier 104 generatesan amplified microphone signal in differential or balanced format acrossthe positive and negative outputs Vop, Von based on the single-endedmicrophone signal at the input 101 b. The unity gain buffer stage 1001may exhibit an extremely high input impedance to avoid loading thetransducer output in view of the potentially very high output impedanceof the transducer as discussed above. If the transducer includes thepreviously discussed capacitive transducer 102 the input impedance ofthe unity gain buffer stage 1001 may be larger than 100 MΩ, such aslarger than 1 GΩ. The differential or balanced microphone voltage at thepositive and negative outputs Vop, Von is applied to a differentialinput of the analog-to-digital converter for conversion into thesingle-bit or multibit digital microphone signal as discussed above.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, different othercomponents. It is to be understood that such depicted architectures aremerely exemplary, and that in fact many other architectures can beimplemented which achieve the same functionality. In a conceptual sense,any arrangement of components to achieve the same functionality iseffectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “operably connected,” or“operably coupled,” to each other to achieve the desired functionality,and any two components capable of being so associated can also be viewedas being “operably couplable,” to each other to achieve the desiredfunctionality. Specific examples of operably couplable include but arenot limited to physically mateable and/or physically interactingcomponents and/or wirelessly interactable and/or wirelessly interactingcomponents and/or logically interacting and/or logically interactablecomponents.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.).

It will be further understood by those within the art that if a specificnumber of an introduced claim recitation is intended, such an intentwill be explicitly recited in the claim, and in the absence of suchrecitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to inventions containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should typically be interpreted to mean “atleast one” or “one or more”); the same holds true for the use ofdefinite articles used to introduce claim recitations. In addition, evenif a specific number of an introduced claim recitation is explicitlyrecited, those skilled in the art will recognize that such recitationshould typically be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, typically means at least two recitations, or two or morerecitations).

Furthermore, in those instances where a convention analogous to “atleast one of A, B, and C, etc.” is used, in general such a constructionis intended in the sense one having skill in the art would understandthe convention (e.g., “a system having at least one of A, B, and C”would include but not be limited to systems that have A alone, B alone,C alone, A and B together, A and C together, B and C together, and/or A,B, and C together, etc.). In those instances where a conventionanalogous to “at least one of A, B, or C, etc.” is used, in general sucha construction is intended in the sense one having skill in the artwould understand the convention (e.g., “a system having at least one ofA, B, or C” would include but not be limited to systems that have Aalone, B alone, C alone, A and B together, A and C together, B and Ctogether, and/or A, B, and C together, etc.). It will be furtherunderstood by those within the art that virtually any disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be understood toinclude the possibilities of “A” or “B” or “A and B.” Further, unlessotherwise noted, the use of the words “approximate,” “about,” “around,”“substantially,” etc., mean plus or minus ten percent.

The foregoing description of illustrative embodiments has been presentedfor purposes of illustration and of description. It is not intended tobe exhaustive or limiting with respect to the precise form disclosed,and modifications and variations are possible in light of the aboveteachings or may be acquired from practice of the disclosed embodiments.It is intended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed is:
 1. A microphone assembly comprising: a capacitivetransducer having first and second spaced-apart plates and configured toconvert sound into an electrical signal at a transducer output; aforward signal processing path comprising: a signal conditioning circuitcomprising an input node connected to the transducer output for receiptof the electrical signal, the signal conditioning circuit configured toamplify or buffer the electrical signal; and an analog-to-digitalconverter (ADC) configured to receive, sample and quantize the amplifiedor buffered electrical signal to generate a digital signal; and afeedback signal processing path comprising: a pulse modulator configuredto generate a digital control signal based on the digital signalobtained from the forward signal processing path; and a currentconverter circuit having an input coupled to the pulse modulator andconfigured to generate a sequence of variable current pulses based onthe digital control signal, an output of the current converter circuitcoupled to one of the first and second spaced-apart plates of thetransducer, wherein the variable current pulses add charge to, orsubtract charge from, the capacitive transducer such that lowfrequencies of the electrical signal are suppressed before theelectrical signal is input to the signal conditioning circuit.
 2. Themicrophone assembly of claim 1, wherein the feedback signal processingpath further comprises a digital loop filter disposed between theforward signal processing path and the pulse modulator, wherein theelectrical signal at least partially reduces low frequency overload anddistortion of the signal conditioning circuit.
 3. The microphoneassembly of claim 2, wherein the ADC comprises: a sigma-delta modulatorconfigured to generate an N-Bit digital signal at a first samplingfrequency, wherein N>0; a decimator between the sigma-delta modulatorand the digital loop filter, the decimator configured to convert theN-Bit digital signal into an M-Bit digital signal at a second samplingfrequency, wherein the second sampling frequency is lower than the firstsampling frequency and wherein M>1; and wherein the M-bit digital signalis input to the digital loop filter.
 4. The microphone assembly of claim2, wherein the digital loop filter is a low pass filter comprising afixed or configurable transfer function.
 5. The microphone assembly ofclaim 2, further comprising a housing having a port, wherein thetransducer is a microelectromechanical systems (MEMS) transducer, andwherein the transducer is disposed in the housing.
 6. The microphoneassembly of claim 5, wherein the transducer has a capacitance between0.5 pF and 10 pF.
 7. The microphone assembly of claim 2, furthercomprising a command and control interface for receipt of filterconfiguration data from a host processor, wherein the digital loopfilter is programmable with the filter configuration data.
 8. Themicrophone assembly of claim 2, wherein the feedback signal processingpath further comprises a noise-shaping quantizer disposed between thedigital loop filter and the pulse modulator.
 9. The microphone assemblyof claim 1, wherein the output of the current converter circuit isdirectly connected to one of the first and second spaced-apart plates ofthe transducer.
 10. The microphone assembly of claim 1, furthercomprising a housing comprising a sound port, the housing enclosing thetransducer, the signal conditioning circuit, the ADC, the pulsemodulator, and the current converter circuit.
 11. The microphoneassembly of claim 10, wherein the pulse modulator is a pulse width andpulse amplitude modulator.
 12. The microphone assembly of claim 10 incombination with a portable communication device.
 13. The microphoneassembly of claim 10, wherein the input node of the signal conditioningcircuit is DC-coupled to the transducer output.
 14. A semiconductor diecomprising: a forward signal processing path comprising: a conditioningcircuit comprising an input node connectable to a capacitivemicroelectromechanical systems (MEMS) transducer having first and secondplates, the conditioning circuit configured to amplify or buffer ananalog electrical signal output by the transducer when connectedthereto; and an analog-to-digital converter (ADC) configured to receive,sample and quantize the amplified or buffered electrical signal togenerate a digital signal; and a feedback path comprising: a pulsemodulator configured to generate a control signal based on the digitalsignal obtained from the forward signal processing path; and a currentconverter circuit having an input coupled to the pulse modulator andconfigured to generate a sequence of variable current pulses based onthe control signal, wherein the variable current pulses add charge to,or subtract charge from, the capacitive transducer such that lowfrequencies of the electrical signal are suppressed before theelectrical signal is input to the conditioning circuit when an output ofthe current converter circuit is coupled to one of the first or secondplates of the transducer.
 15. The semiconductor die of claim 14, whereinthe feedback path further comprises a digital loop filter disposedbetween the ADC and the pulse modulator, and a noise-shaping quantizerdisposed between the digital loop filter and the pulse modulator,wherein the electrical signal at least partially reduces low frequencyoverload and distortion of the forward signal processing path.
 16. Amicrophone assembly comprising: a capacitive microelectromechanicalsystems (MEMS) transducer having first and second electrodes, thetransducer configured to generate an electrical signal in response todetecting a change in air pressure, the electrical signal includingfrequencies in a first frequency range and frequencies in a secondfrequency range, the first frequency range higher than DC and the secondfrequency range higher than the first frequency range; a processingcircuit coupled to the transducer, the processing circuit configured togenerate an output audio signal based on the electrical signal generatedby the transducer, the processing circuit comprising an analog todigital converter configured to generate a digital signal; and acompensation circuit coupled to the processing circuit and the first orsecond electrode of the transducer, the compensation circuit configuredto generate a variable feedback signal based on the digital signal andprovide the variable feedback signal to the first or second electrode ofthe transducer; wherein the variable feedback signal adds charge to, orsubtracts charge from, the capacitive transducer such that frequenciesin the first frequency range are suppressed before the electrical signalis applied to the processing circuit.
 17. The microphone of claim 16,wherein the first frequency range includes frequencies below a humanaudible range of frequencies and the second frequency range includesfrequencies within the human audible range, and wherein the compensationcircuit is configured to cancel frequencies below the human audiblerange from the electrical signal before the electrical signal is appliedto the processing circuit.
 18. The microphone assembly of claim 17,further comprising a housing, the transducer, the processing circuit,and the compensation circuit disposed in the housing and an output ofthe processing circuit coupled to an electrical interface contact of themicrophone assembly.
 19. The microphone assembly of claim 18, thecompensation circuit comprising a pulse width and amplitude modulatorconfigured to generate a control signal based on the digital signal; acurrent converter circuit configured to generate a sequence of variablecurrent pulses based on the control signal, wherein the variablefeedback signal includes the variable current pulses.
 20. The microphoneassembly of claim 19, the compensation circuit further comprising: a lowpass digital loop filter disposed between the processing circuit and thepulse width and amplitude modulator; and a noise-shaping quantizerdisposed between the low pass digital loop filter and the pulse widthand amplitude modulator; wherein the electrical signal at leastpartially reduces low frequency overload and distortion of theprocessing circuit.